Field of the Instant Invention
The present invention relates most generally to power supplies for professional and consumer electronic devices, and more particularly to a circuit or circuit stage to improve power supply efficiency and reliability, and still more particularly to a device to improve instantaneous current flow into an AC/DC power supply commonly used in consumer electronic devices.
Background Discussion
It is well understood that an AC electromagnetic wave propagates through a metal conductor at velocities ranging between 60 to 90 percent of the speed of light. What is less well known (at least less well appreciated by lay persons) is that the current —specifically the individual charge carriers (i.e., the electrons)—do not move (or drift) at anywhere near that speed. In physics this phenomenon is referred to as drift current. Although the electromagnetic wave propagates at fractional light speeds, the drift current propagates at speeds of millimeters per second or less, depending upon the specific peak voltage, total circuit impedance, and the power line frequency.
In a typical power supply for an electronic device, the power supply draws current from the power line in pulses. Current is only drawn at the positive and negative peaks of the AC voltage waveform. The storage capacitors for the power supply maintain a relatively stable DC voltage, and the rectification diodes only turn on when the AC waveform voltage exceeds the stored voltage level in the capacitor array. When the rectification diodes do turn on there is high surge of current drawn from the AC power line. The sudden demand for instantaneous current from the power supply can exceed the capacity of the power line to deliver current efficiently due to inductive reactance and simple resistance of the line. The inductance of the power line conductors will cause a significant rise in impedance to the change of instantaneous current when the power supply rectifiers switch on. This pulsating current draw creates high order voltage and current harmonics, which, in turn, create distortion harmonics as high as the 50th harmonic of the power line frequency.
In large power distribution systems this problem causes a “power factor” degradation. The voltage phase leads the current phase, which reduces the effective power delivery to the load or power supply. A common solution applied to the power factor problem is to install large arrays of storage capacitors on the AC power line. Disadvantageously, these are large, heavy, generate heat, and have a relatively short “mean time between failure” rate.
What is needed is a solution that stores energy “locally”, near the power supply. Such as solution should allow the power supply to draw current pulses at a rate higher than would normally be allowed by the inductive reactance of the power line.